Conventionally, one including plural boilers for a single steam turbine is known as a steam turbine plant. Besides, one including a superheater and a reheater is known as the boiler of the steam turbine plant. As for the steam turbine plant as stated above, a steam flow rate required by a steam turbine is small from an aeration start for the steam turbine at a start-up until reaching a predetermined load. Accordingly, steam is supplied from a part of the boilers to the steam turbine from the aeration start for the steam turbine until reaching the predetermined load. Hereinafter, a part of the boilers supplying the steam to the steam turbine at the aeration start time is referred to as an aeration boiler. Besides, a remaining boiler which does not supply the steam to the steam turbine at the aeration start time is referred to as a standby boiler.
After reaching the predetermined load, steam of the standby boiler is merged with steam of the aeration boiler, and supplied to the steam turbine (Tie-in). Conventionally, steam pressures (reheat steam pressures) supplied from the reheaters of the aeration boiler and the standby boiler at the Tie-in time are coincident. There is a problem as described below in a conventional technology as stated above.
For example, a bypass pipe guiding reheat steam from the standby boiler to a steam condenser is provided at a boiler side so that the reheat steam from the standby boiler is not supplied to the steam turbine. The reheat steam pressure of the standby boiler is set to be rather high from the aeration start time so that a valve capacity of a bypass valve provided in a middle of the bypass pipe does not become large. Besides, the reheat steam pressure of the aeration boiler is set to be high in response to the standby boiler.
However, at a steam turbine side, a windage loss is easy to occur because a high-pressure turbine is not able to work sufficiently at the aeration start time. In particular, when a steam pressure in a vicinity of a final stage is high, a temperature of a blade of the final stage is easy to increase exceeding an allowance together with the windage loss. Accordingly, there is a possibility of an occurrence of a serious trouble such as a contact between the blade and a static part.
The steam from the high-pressure turbine has been bypassed to the steam condenser up to now so as to satisfy a requirement of the steam turbine side while satisfying the requirement of the boiler side. For example, a bypass pipe is provided from a middle of a low-temperature reheat steam pipe connected to an outlet of the high-pressure turbine to be connected to the steam condenser. However, it is preferable to satisfy the requirements of the boiler side and the steam turbine side without providing an additional bypass pipe as stated above. Specifically, it is preferable to suppress the windage loss and a temperature increase at the high-pressure turbine while making the valve capacity small.
Besides, when the reheat steam pressures of both the aeration boiler and the standby boiler are set to be high, a fuel consumption amount becomes large. Accordingly, it is required to start-up the steam turbine plant without setting the reheat steam pressures of the aeration boiler and the standby boiler high.
As stated above, it is required to suppress the valve capacity of the bypass valve provided at the bypass pipe connecting the standby boiler and the steam condenser as for the conventional steam turbine plant. Besides, to suppress the valve capacity, it is required not to provide the additional bypass pipe. Further, to suppress the valve capacity, it is required to suppress the fuel consumption amount.